As apparent from FIGS. 1a, 1b and 1c, a very short-lived member occurs in each of the three natural decay chains.
In the decay chain of U-238 (FIG. 1a) a product resulting from the decay of Rn-222 is the nuclide Bi-214 which, emitting .beta. radiation, decays into Po-214 which in turn decays with a half-life of 0.16 ms and emits .alpha. radiation.
In the decay chain of U-235 (FIG. 1b) the decay product of Ra-223 is the nuclide Rn-219 which, emitting .alpha. radiation, decays into Po-215 which in turn decays with a half-life of 1.8 ms and emits .alpha. radiation.
In the decay chain of Th-232 (FIG. 1c) a product resulting from the decay of Ra-224 is the nuclide Bi-212 which, emitting .beta. radiation, partly decays into Po-212 which in turn decays with a half-life of 0.3 .mu.s and emits .alpha. radiation.
During some of the decays gamma radiation is additionally emitted.
Methods for counting nuclides of these decay chains by the principle of delayed coincidence employ the quick succession of decays of a mother nuclide Bi-214, Rn-219 or Bi-212 and its short-lived daughter nuclide Po-214, Po-215 or Po-212.
The successive decays cause pairs of quickly succeeding pulses to arise in the counting apparatus. The coincidence of these pulses within a brief time interval makes it possible to distinguish other pulses distributed statistically over the counting time provided their mean distance is substantially greater than the coincidence time interval. Such pulses are caused e.g. by other nuclides in the counting sample, by external gamma radiation or by cosmic radiation. The time difference between the pulses of a pair permits distinction from other coincidence events.
The probability of detecting a pair is proportional to the product of the probabilities of detection for the individual decay events. Detectors with high probabilities of detection are therefore preferable to those with low probabilities of detection. Liquid scintillators are particularly suitable since they have probabilities of detection of almost 100% for the .alpha. decays and high-energy .beta. decays taking place in them.
A method for liquid scintillation counting of nuclides of the natural decay chains by the principle of delayed coincidence is already known (G. Assaf and J. R. Gat, "Direct determination of short-lived radon daughter products on air filters by liquid scintillation counting using a delayed-coincidence technique," Nuclear Instruments and Methods 49 (1967), 29-37).
It is intended for counting decay products of Rn-222 in air. According to this literature, an air filter is immersed in a liquid scintillator cocktail after air has been drawn through the filter. The scintillations caused by the .beta. and .alpha. particles emitted during the decays of the collected atoms are converted into logical pulses by an assembly comprising a photomultiplier followed by a discriminator. Because the scintillator perfuses the air filter and surrounds it on all sides, almost a 4 counting geometry is obtained for the emitted particles. Because the air filter becomes transparent in the scintillator, the light from flashes produced on the side of the filter facing away from the photomultiplier can also reach the photomultiplier.
The level of the discriminator is set as low as possible so that during .beta. decay of Bi-214, the mother nuclide of the short-lived member Po-214, the emitted electrons are detected down to an energy as low as possible. A first counter counts all logical pulses at the output of the discriminator.
To count the decays of the short-lived nuclide Po-214 by the principle of delayed coincidence, the logical pulses are processed at the output of the discriminator as follows, as is indicated particularly clearly by FIG. 2 of this literature. A unit called a "delay gate" in the cited paper produces a "gate open" pulse after each "random gate opening" discriminator pulse. This gate open pulse begins with a delay time delta t.sub.d (d=delay) after the discriminator pulse and has the length delta t.sub.g (g=gate). The term "random gate opening pulses" is used in this paper for all logical pulses at the output of the discriminator which do not occur themselves during a gate open interval. A coincidence unit, whose two inputs are connected with the outputs of the discriminator and the delay gate unit, creates at its output a logical pulse exactly when a pulse occurs at the output of the discriminator during a gate open pulse at the output of the delay gate unit. The pulses at the output of the coincidence unit are counted by a second counter.
According to this paper, the optimal length of the gate open interval is between five and ten times the half-life of Po-214 (0.8 ms or 1.6 ms). The delay time delta t.sub.d is selected as 2 .mu.s, i.e. about 7 half-lives of Po-212 from the decay chain of Th-232. If a discriminator pulse comes from a decay of Bi-212, the mother of Po-212, the atom of the short-lived daughter nuclide Po-212 that was formed during this decay has most probably already decayed at the start of the gate open interval, i.e. the delay time prevents its decay from being falsely recorded as a Po-214 decay.
This paper points out that the described counting method with accordingly shorter time intervals delta t.sub.d and delta t.sub.g is also suitable for counting the decays of the nuclide pair Bi-212/Po-212 from the decay chain of Th-232.
The counting method described can be used for counting those samples consisting of a scintillator into which the nuclides to be counted have been introduced in a way other than via filters.
However, when high or very low activities are to be measured, or when the photon yield varies in the counting samples of a series, the method described in this paper has the following weaknesses:
(1) When samples are counted at a high count rate a large part of the total counting time is occupied by gate open intervals. This results in a nonnegligible probability that those discriminator pulses which do not come from a Po-214 decay will also occur during a gate open interval and be recorded in the second counter as decays of Po-214. The random delayed-coincidence events already necessitate elaborate numerical corrections at total count rates below 100 pulses per second.
(2) When the activity of the nuclides to be counted is small in a counting sample as compared to the background count rate U, the rate of random delayed-coincidence events is equal to U.sup.2 .multidot./delta t.sub.g. At a background count rate of e.g. 0.2 pulses/s and delta t.sub.g =1 ms the resulting theoretical rate of random delayed-coincidence events is 4.multidot./10.sup.-5 pulses/s. The true delayed-coincidence events caused by secondary radiation of single cosmic particles additionally appear in the second counter. This makes it difficult to count Ra-226 or Rn-222 activities in the range of a few 10.sup.-5 Bq.
(3) When the photon yield of two counting samples is different a different fraction of pulses which are produced by the .beta. decays of Bi-214 exceeds the discriminator level, i.e. the counting efficiency is different for the counting samples. The known counting method does not include any possibility of recognizing differences in the photon yield.
The weaknesses of the method according to Assaf and Gat are essentially due to the fact that no use is made of the energy information of the delayed-coincidence decay.
Further methods for liquid scintillation counting of nuclides of the natural decay chains which use only the time correlation of decay events and not the energy information of these decay events are described in the following literature:
P. Cross, G. W. McBeth and H. P. Primmington, "Rapid identification and radioassay of picocurie quantities of the alpha-emitting series," Nuclear Instruments and Methods 125 (1975), 425-427;
P. Cross and G. W. McBeth, "Absolute determination of trace quantities of the Ra-226 series by time interval analysis," Nuclear Instruments and Methods 137 (1976), 135-139.
Unlike these methods, the detectors used most frequently for counting nuclides of the natural decay chains, namely surface-barrier detectors, are specially designed for a high energy resolution during detection of .alpha. particles. The disadvantages of this type of detector are that:
(1) the maximum probabilities of detection attained are 50%, and
(2) special methods must be used to produce extremely thin counting samples that permit a largely undisturbed emission of .alpha. particles.
It is particularly difficult to process radium quantitatively into a thin counting sample, one of the essential applications of the invention being to count radium.
A compromise is liquid scintillation .alpha. spectroscopy. It combines a probability of detection of 100% for the .alpha. decays in the scintillator with a moderate energy resolution. It has gained importance since it has become possible to distinguish by pulse-shape analysis between .alpha. decays, on the one hand, and .beta. decays and background events, on the other (W. J. McDowell, "Alpha liquid scintillation counting: past, present and future" in "Liquid scintillation counting, recent applications and development", Vol. I, Physical aspects, Academic Press, New York 1980; T. Oikari, H. Kojola, J. Nurmi and L. Kaihola, "Simultaneous counting of low alpha and beta particle activities with liquid scintillation spectrometry and pulse-shape analysis," Appl. Radiat. Isot. 38 (1987), 875-878). A precondition for successful pulse-shape analysis is the absence of certain quenching pollutants in the scintillator and thus special care during preparation of the samples.
Information on both, the energy and the time correlation of detector events, is provided by so-called "time-of-event" counting methods. In these methods the time an event occurs together with the energy information and any additional information is stored successively event by event. Such methods have been applied up to now for counting with low-level gas counters and are described e.g. in the following literature:
L. A. Currie, R. W. Gerlach, G. A. Klouda, F. C. Ruegg and G. B. Tompkins, "Miniature signals and miniature counters: accuracy assurance via microprocessors and multiparameter control techniques, "Radiocarbon 25 (1983), 553-564;
L. Kaihola, H. Polach and H. Kojola, "Time series analysis of low level gas counting data," Radiocarbon 26 (1984), 159-165;
L. Kaihola, H. Kojola, H. Polach, P. Mantynen, J. Tervahauta and E. Soini, "Low level gas multicounter for C-14 dating of small samples: electronic, numerical and shielding optimization," Third Nordic Conference on the application of scientific methods in archaeology, Mariehamm, Aland, Finland,
The methods described by Kaihola et al. work with a time resolution of 10 or more ms, i.e. they are not designed for detecting Rn-222 contaminations in the counting gas, for which Assaf and Gat consider a time resolution in the range of 1 ms to be expedient. Instead, Kaihola, Polach and Kojola expressly point out that they used time series analysis only to detect periodic effects.
However, time-of-event counting methods are basically also very well-suited for liquid scintillation counting of the nuclides stated in the invention. A disadvantage of these methods is the high electronic elaborateness which is necessary in particular if the information should already be presented clearly to the experimenter during a measurement. Also, such methods can be installed only with great effort in the customary liquid scintillation spectrometers which are equipped with multichannel analyzers.